There's a mix here of Vanilla and TTT/Dark Codex (use the ModFinder Mod to manage your mods and keep them up to date. Very easy and highly recommended, tho I'm only using about half of them. You've got your choice.) and I know some people prefer Vanilla so I'll try to evaluate them separately.
And some of the Vanilla ones have been modified, as with Ascendant Summons. Combine this with Augment Summons and you've got a force on your hands.
Vanilla:
Last Stand: pretty much mandatory for Unfair, highly recommended for Hard. Taking this first relieves a lot of headaches but adds 1/2 a difficulty level itself because you're missing your first Mythic. Much more satisfying to do that tho than having to deal with Unfair double damage nat 20s all game.
Inspirational Leader: best scaling Mythic bar none, but only need on one character. I usually try to put on a non-caster and not really needed until M3 or M4 but after that it's amazing, for all three modes.
Ever Ready: highly underrated for any group that generates AoOs, typically via Trips (melee-heavy)/Outflank crit-chains or Fear effects (ranged). I've also gotten a lot with Disarms (Maraliths FTW). Saves a Feat on Combat Reflexes and both AB and Damage bonuses really add up fast. Mythics that add AB + Damage > just Damage in most cases.
Modded:
Barrage: this is what TTT has had for awhile. Guessing this is what Owlcat implemented? Needs a spell to apply the mark, but anything can cash it.
Armor Master: Armor Training for classes that don't get it. Except for the speed. Good for Mounted Pals to get their Mobility to a decent level I guess. Cavs get a special ability that bypasses penalty to Mobility from Heavy. Niche mostly although maybe can do something with the Arcane side?
Armored Might: Archmage for Armored classes. Game changer. Once you have this Armor Master starts looking a lot better.
Vanilla:
Leading Strike: RTwP only but really good there since you interlace attacks and it triggers all over the place and also on Outflank crit-chains. Favorite for Reg.
Rupture Restraints/Unstoppable: Very good if you're playing limited reloads to prevent wipes. I started out liking Rupture since it deals with more things but there are mobs that spam Shaken that can burn through your activations. Still probably better if you've got Seelah to give Fear Immunity. Otherwise Unstoppable is probably lower maintenance.
Modded:
Limitless x: there are two versions of these, and they have them for about every class that has a resource pool. The more powerful ones require two Mythics (or a Mythic and a Feat) while things like the Warpriest one only take one. If your class has one of these features you want to grab it ASAP since the earlier you get it the better they are (unlike most scaling Mythics).
Vanilla:
Ranging Shots: the modded version here doesn't go back down on hits but that doesn't come up much when you need it since the problem it helps solve is missing. Since Ranged doesn't get to Flank, Outflank, or Crusader's Edge it's down 6 compared to melee and this comes up a lot (especially since the strength of Ranged is all the full attacks it gets so you're trying to land iteratives more often and fighting through the Rapid malus as well). Since it's capped by Mythic Rank you can wait on it but I try to pick it up on all Ranged characters by Mythic 4 so I can have it in time for Blackwater/Fane etc...
Abundant Bane: if you're going low rest Bane is going to be tough to keep up much but if you take the rests they give you this should tide you over for most tough fights. Obv much better for Aeon. Cam gets Bane Weapon at lvl 8 so something to keep in mind for her. Another + AB/damage Mythic.
Mythic Charge: if you're Pouncing like
Haplo this could be good. Would have to consult the master on this one. Never used it.
Mythical Beast: modded adds Ghost Touch which makes this good earlier than usual. DR bypass OK I guess but with Relic plus multiple attack modes of Natural Attacks not that big of a deal. Something you'll want to pick up once it scales to 4 or maybe 6.
The Bigger They Are: I've tried this and I think it's just too niche. Good vs Wurms and Dragons but not things like Playful.
Second Mystery: even better than Second Spirit as long as you're playing an Archetype with enough Revelations to take advantage of it. I've been Mounting up Daeran ASAP with Nature (also gives him Barkskins) and been very happy with it. Battle and Ancestors also been very solid.
Modded:
Cursing Gaze: There's also one that removes the once per 24 hr restriction on Hexes so tough choice. This one can really unclog your action econ so definitely worth a look.
Vanilla:
Everlasting Judgement: completely changes the way Inquisitor plays. Judgements are really good and kick up to even better at lvl 10. Almost a must have, especially if you're low rest which would otherwise make the class almost unplayable since it relies so heavily on Bane Weapon, Judgments, and rnd/lvl spells.
Cleaving Shot: main reason to have Ranged characters in the first place and to put in the effort to up their crits. Even without the crits they're often finishing off stragglers making the AoE trigger OFTEN.
Distracting Shots: this stacks with itself so if you have a ranged character with high AB (like two Axe Wend) and a lot of attacks you can really go off. I try to get all three Ranged Mythics on Ranged characters since they all solve problems that Ranged parties typically confront. It's not quite as good as it looks since Ranged typically has lower AB than melee but once you have this it makes things like advantage effects on ranged attacks force multipliers.
Modded:
That should be the Mythic that removes the 24 hr limit on Hexes that I mentioned before. Another couple examples of the Limitless Mythics. Incense will be particularly good once they implement that class since it doesn't get Lingering. Limitless Arcane Pool of course is just absurd.
EDIT: somehow I left off Instill Vigor (the one that gives AoE Mythic Bonus if you damage your own team). It's worth unlocking on one character.
